APW7142 datasheet(13/22 Pages) ANPEC

13 / 22 page

Rev. A.6 - Nov., 2010

APW7142

www.anpec.com.tw

Function Description

Over-Voltage Protection (OVP)

The over-voltage function monitors the output voltage by

FB pin. When the FB voltage increases over 123% of the

reference voltage due to the high-side MOSFET failure or

for other reasons, the over-voltage protection comparator

will force the low-side MOSFET gate driver high. This ac-

tion actively pulls down the output voltage and eventually

attempts to blow the internal bonding wires. As soon as

the output voltage is within regulation, the OVP compara-

tor is disengaged. The chip will restore its normal

operation. This OVP scheme only clamps the voltage

overshoot, and does not invert the output voltage when

otherwise activated with a continuously high output from

low-side MOSFET driver - a common problem for OVP

schemes with a latch.

Over-Temperature Protection (OTP)

The over-temperature circuit limits the junction tempera-

ture of the APW7142. When the junction temperature ex-

ceeds T

J = +150

oC, a thermal sensor turns off the both

power MOSFETs, allowing the devices to cool. The ther-

mal sensor allows the converters to start a start-up pro-

cess and to regulate the output voltage again after the

junction temperature cools by 40oC. The OTP is designed

with a 40oC hysteresis to lower the average T

J during

continuous thermal overload conditions, increasing life-

time of the APW7142.

Enable/Shutdown

Driving EN to the ground initiates a soft-stop process and

then places the APW 7142 in shutdown. W hen in

shutdown, after the soft-stop process is completed, the

internal power MOSFETs turn off, all internal circuitry shuts

down and the quiescent supply current reduces to less

than 3

µA.

VIN Power-On-Reset (POR)

The APW7142 keeps monitoring the voltage on VIN pin to

prevent wrong logic operations which may occur when

VIN voltage is not high enough for the internal control

circuitry to operate. The VIN POR has a rising threshold of

4.1V (typical) with 0.5V of hysteresis.

During startup, the VIN voltage must exceed the enable

voltage threshold. Then, the IC starts a start-up process

and ramps up the output voltage to the voltage target.

Digital Soft-Start

The APW7142 has a built-in digital soft-start to control the

rise rate of the output voltage and limit the input current

surge during start-up. During soft-start, an internal volt-

age ramp (V

RAMP), connected to one of the positive inputs

of the error amplifier, rises up from 0V to 0.95V to replace

the reference voltage (0.8V) until the voltage ramp reaches

the reference voltage.

During soft-start without output over-voltage, the APW7142

converter’s sinking capability is disabled until the output

voltage reaches the voltage target.

Digital Soft-Stop

At the moment of shutdown controlled by EN signal, un-

der-voltage event or over-temperature protection, the

APW7142 initiates a digital soft-stop process to discharge

the output voltage in the output capacitors. Certainly, the

load current also discharges the output voltage.

During soft-stop, the internal voltage ramp (V

RAMP) falls

down rises from 0.95V to 0V to replace the reference

voltage. Therefore, the output voltage falls down slowly at

the light load. After the soft-stop interval elapses, the soft-

stop process ends and the the IC turns on the low-side

power MOSFET.

Output Under-Voltage Protection (UVP)

In the operational process, if a short-circuit occurs, the

output voltage will drop quickly. Before the current-limit

circuit responds, the output voltage will fall out of the re-

quired regulation range. The under-voltage continually

monitors the FB voltage after soft-start is completed. If a

load step is strong enough to pull the output voltage lower

than the under-voltage threshold, the IC shuts down

converter’s output.

The under-voltage threshold is 50% of the nominal out-

put voltage. The under-voltage comparator has a built-in

µs noise filter to prevent the chips from wrong UVP shut-

down being caused by noise. The under-voltage protec-

tion works in a hiccup mode without latched shutdown.

The IC will initiate a new soft-start process at the end of

the preceding delay.

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